The present invention relates to a method for determining cardiac output or total peripheral resistance from the measurement of an arterial pressure waveform and in particular from a non invasive measurement of peripheral circulation.
While it is preferred to use a non-invasive technique to measure the arterial pressure waveform, an invasive technique such as the use of an intra-arterial catheter can also be used. While such a technique is more accurate its use is more restricted to qualified personnel and has more limited use.
Intra arterial catheters are in common use during major surgery. These are commonly attached to pressure transducers and the electrical signal is processed to display the pressure waveform as a trace on a visual display monitor. The display indicates systolic, diastolic, mean arterial pressure and heart rate.
At present the most reliable and accepted method of measuring cardiac output for surgical patients derives the cardiac output from a thermodilution technique using the insertion of a pulmonary artery catheter. The procedure is potentially dangerous as the catheter must be passed through the right heart for correct placement. In children it is considered that the danger of the technique outweighs any benefits from the information received.
In each of U.S. Pat. No. 5,535,753 (Petrucelli et al) and U.S. Pat. No. 5,647,369 (Petrucelli et al), a non-invasive technique measures cardiac output using an unmodified Windkessel circuit model representing arterial compliance as a single lumped capacitance. In U.S. Pat. No. 5,535,753 cardiac output is calculated from the mean flow in diastole multiplied by the heart rate with mean flow determined by RMS averaging systolic xe2x80x9ccurrentxe2x80x9d over the diastolic period. Systolic current is calculated from a model using a LC charge pump circuit for the systolic portion of the cardiac cycle. In U.S. Pat. No. 5,647,369 (Petrucelli et al) provide an allegedly improved measure of cardiac output in a known equation involving a function of pulse pressure, heart rate and compliance. by representing compliance as a function of pulse pressure, age, height and/or weight.
In U.S. Pat. No. 5,836,884 (Chio), cardiac output or peripheral resistance is calculated from a non-invasive cuff pressure method measuring systolic, diastolic and mean arterial pressure and employing formulae using diastolic flow velocity calculated from determinants of the pressure waveform.
Non invasive techniques for determining cardiac output include Doppler ultrasonography using an oesophageal probe and echo cardiography using a probe on the chest wall. The equipment for these techniques is bulky, expensive and unsuited to routine theater use where space is of a limited nature especially during major surgery.
Other techniques for measuring cardiac output use expired gas analysis using a variety of gases. Most of these techniques are experimental only and have not been accepted for general clinical use.
Another form of measurement involves a dye dilution technique but it is of limited use as it does not allow frequent repeated measurement.
The present invention seeks to overcome the disadvantages in the prior art and to provide a simpler and easier method for obtaining cardiac output by measuring the arterial pressure waveform and in turn providing measurement of cardiac output or total peripheral resistance.
According to the invention there is provided a method for deriving cardiac output in a patient including the steps of: measuring continuously the arterial pressure at a point within the cardiovascular system to derive an arterial pressure waveform; determining the mean arterial pressure from said arterial pressure waveform; determining the compliance of the arterial system for the patient; determining the time constant of the arterial system from the arterial pressure waveform and deriving the mean cardiac output as the product of the mean arterial pressure and compliance divided by said time constant.
The time constant of the arterial system corresponds to the time from the dicrotic notch to the time where the slope of the arterial pressure waveform at the dicrotic notch intersects the time axis, equivalent to extrapolation of the waveform to zero arterial pressure. The dicrotic notch corresponds to the pulse in the arterial pressure waveform produced by aortic valve closure. The arterial pressure waveform after the dicrotic notch can be represented by an exponential decay curve.
The compliance of the arterial system is obtained from a set of nomograms which are normalised and based on the weight, height, age and sex of a patient.
Other factors can be determined from these measurements including stroke volume which is the mean cardiac output divided by the heart rate. Cardiac contractility can be derived from the positive slope of the arterial pressure waveform, maximum contractility being taken from the steepest part of that slope which can then be displayed both as a rate of change of pressure or, when the stroke volume is known, as the rate of change of volume. Other determinations that can be provided by the current measurement techniques include the ejection period which is the interval from the commencement of the upswing of the arterial pressure waveform to the dicrotic notch.
Other parameters can also be determined from the above measurements, the explanation of which is made below.